Fluid directing vanes, such as those used in the turbine modules of gas turbine engines, are exposed to hot, gaseous combustion products. Various measures are taken to protect the vanes from the damaging effects of the hot gases. These include making the vane of temperature tolerant nickel or cobalt alloys, applying thermal barrier coatings to the vanes, and cooling the vanes with relatively cool, pressurized air extracted from the engine compressor.
Conventional cooling techniques include impingement cooling. An impingement cooled vane has an internal cavity and a sheet metal coolant insert or baffle residing in the cavity but spaced a small distance from the cavity wall. The space between the baffle and the cavity wall is referred to as an impingement space. The baffle, which is usually made of a nickel alloy, is welded to the vane near the spanwise extremities of the vane. The weld joint secures the baffle to the vane and also seals the spanwise extremities of the impingement cavity. Numerous impingement cooling holes perforate the baffle. During engine operation, coolant enters the interior of the baffle and then flows through the impingement cooling holes, which divide the coolant into a multitude of high velocity coolant jets. The coolant jets impinge on the cavity wall to keep the wall cool. The coolant then discharges from the impingement cavity, customarily by way of coolant discharge passages that penetrate the cavity wall.
Despite the many merits of the above mentioned alloys, coatings and cooling techniques, it is desirable to further improve the temperature tolerance of turbine engine vanes to extend their useful life or to allow the engine to operate at higher internal temperatures, which improves engine performance. One way to improve the temperature tolerance is to construct the vanes of a refractory material. Refractory materials include refractory metal alloys (such as molybdenum and niobium alloys) ceramics, and compositions comprising intermetallic compounds. However these materials are susceptible to cracks because they are brittle at some or all temperatures.
In addition, although refractory materials exhibit better temperature tolerance than nickel or cobalt alloys, it may still be necessary to employ impingement cooling using a conventional metal baffle as already described. A conventional metal baffle is desirable, even in a vane made of refractory material, for at least two reasons. First, conventional baffle alloys have a higher coefficient of thermal expansion than do the refractory materials, but are exposed to lower temperatures during engine operation. Consequently, the thermal response of the conventional metal baffle will be compatible with that of the refractory vane. Second, a conventional metal baffle, unlike a refractory baffle, can be perforated with impingement cooling holes without suffering any appreciable loss of structural integrity. Unfortunately, a conventional metal coolant baffle cannot be welded to a refractory vane in order to secure the baffle to the vane and seal the ends of the impingement cavity. In principle, the problem of sealing the ends of the impingement cavity could be overcome by using a seal made of a compliant material. In practice, however, such seals are incapable of withstanding the extreme temperatures and/or the mechanical abuse (e.g. vibration and chafing) encountered in a turbine engine. Moreover, even if a suitable seal material were available, it would not, by itself, address the problem of securing the metal baffle to the ceramic vane.
What is needed is a coolable, highly temperature tolerant vane assembly that exhibits good crack resistance, is capable of accepting a metal baffle, and is achievable without requiring the use of materials unsuitable for a harsh thermal and mechanical environment.